1. Field of the Invention
The present invention relates generally to detection of echo path changes in echo cancellers, and more particularly to detect the echo path change (EPC) in a full duplex handsfree (FDHF) speakerphone and to mask the increased error due to the echo path change.
2. Description of the Related Art
Two types of echoes are prevalent in modern communication system: line echo and acoustic echo. Line echo arises at a four- to two-wire interface, or “hybrid”, whereas acoustic echo occurs when sound from the telephone handsfree speaker feeds back to the microphone either directly or is reflected off of different surfaces, such as walls, tables, etc. Hybrid echo is created as voice signals are transmitted across the network via the hybrid connection at the two-wire/four-wire public-switched telephone network PSTN conversion points.
The actual amount of signal that is reflected back depends on many factors including the size of the room, the “hardness” of materials that the speaker signal reflects off of, the loudness of the speaker signal, and the distance between the speaker and microphone. The majority of echo is due to the speaker signal being picked up directly by the microphone.
The parameter for defining the effectiveness of the echo attenuation is echo-return loss (ERL). A high ERL indicates a low reflected-signal back to the talker and vice versa.
The effective removal of echo is one key to maintaining and improving perceived voice quality on a call. This has led to intensive research into the area of echo cancellation, with the aim of providing echo cancellers that can reduce acoustic echo, thereby improving overall speech quality. The measure of an echo canceller's ability to remove echo is ERL enhancement (ERLE). To completely remove the echo from the circuit, echo cancellation creates a model of the echo path, synthesizes a replica estimate of the echo, and cancels the echo by subtracting the estimated echo from the true echo. This process allows full-duplex speech between the near and distant callers and results in natural, interactive speech.
Typically, a nonlinear processor (NLP) is activated when the echo canceller has determined that there is no near-end speech, which results in full attenuation of all speech signals (real near-end speech and echo from the far end of the conversation).
It is known in the art to employ adaptive filtering within echo cancellers. In an adaptive filter, the filter coefficients are based, in part, on feedback of filter output. Normalized Least Mean Square (NLMS) adaptive filtering is one method, popular in echo cancellation, to address reflections in the telephony system.
In such echo cancellers, the coefficients of an adaptive filter converge to a certain echo path. Under ideal conditions, a generally acceptable convergence time requires that the echo canceller achieve 27 dB of ERLE (Echo Return Loss Enhancement) in 0.5 sec. Once the coefficients are converged, the echo is cancelled from the input signal. When the echo path changes (i.e. call transfer, conferencing, or a telephone user touching the phone display or keys while a handsfree call is in progress), the echo canceller has to quickly re-converge to the new echo path or else the echo will be perceived by the user. Line echo path changes are infrequent and last a relatively long time. Acoustic echo path changes are more frequent and relatively short in duration. After an acoustic echo path change, the echo path usually returns to the previous state (i.e. after the user stops touching the touchscreen LCD or pushing a button, the echo path is the same as before the user touched the screen or pushed a button).
Consequently, it is known in the art to preserve essential information on the constant portion of the acoustic echo path (or the acoustic feedback through the plastic housing of a telephone) as default coefficients for use at start-up to quickly converge the echo canceller. Examples of this technique are set forth in U.S. Pat. No. 6,768,723 and published Canadian Patent Application 2,451,417. As disclosed in the foregoing prior art, two filters are utilized. The first, a “short” fitter, uses non-adaptive default coefficients (N_short coefficients) to preserve the information of the captured constant echo path. The second, a “long” fitter, uses adaptive filter coefficients (N_long coefficients) for adapting constantly toward the best solution for a current echo path (where N_long>N_short). Each time the convergence of the long filter improves, the first N_short coefficients from the long filter are transferred to the short filter to replace the default coefficients.
Detecting the echo path changes is a difficult problem since the echo resulting from a new echo path and the echo generated by the old echo path from the converged adaptive filter can easily be misinterpreted as double talk.
Prior art solutions to this problem may be found in U.S. Pat. No. 6,035,034 (Trump, Tonu): Double talk and Echo Path Change Detection in a Telephony System, and U.S. Pat. No. 6,226,380 (Heping, Ding): Method of Distinguishing Between Echo Path Change and Double Talk Conditions in an Echo Canceller.
In addressing the foregoing problems, published Canadian patent application no. 2,494,500, sets forth a method for detecting echo path changes that uses the statistics of the echo cancelling behaviour (i.e. signal and performance information), to distinguish between new line and double talk conditions. In terms of speech dynamics, double talk conditions are relatively short in duration, whereas a new line condition remains active. Using a moving counter (referred to in CA 2,494,500 as an Echo Path Change Counter or EPC Counter), an evaluation is made of the probability that the echo canceller behaviour is responding to an echo path change and not a double talk scenario. By monitoring the ERL (Echo Return Loss), ERLE (Echo Return Loss Enhancement), noise levels and signal energies, the Echo Path Change Counter is incremented or decremented. When the counter reaches a predetermined threshold value indicative of sustained poor echo performance, a determination is made that there is a probable new line condition. This echo path change information is then passed to the echo canceller to enable re-convergence.